If you just want clustered virtual machines, check out Ganeti[0]. It's not advertised much, but this piece of software hosts most of Googles internal infrastructure (not public facing stuff).

Unlike Openstack, it has a proper scheduler, and lets you rebalance VMs across hypervisors efficiently. Also unlike Openstack, it can restart VMs if it (or the hypervisor) dies, if you've enabled that.

And completely orthogonal to Openstack, it has very strong consistency guarantees. It's not made to start 1000s of VMs in seconds, since each master node has to agree on all decisions, and each operation typically "locks" the involved hypervisor. On the other hand, I haven't been able to break it once in over six years.

Note that it really just exposes an API and comes with a superb command-line client. Some assembly required.

Source: deployed Ganeti with great success at a billion-euro company, moved on to promising "cloud" project which insisted on using Openstack and promptly quit after a year of fighting obscure bugs (and naive colleagues who did not want to try anything else :)).

(If you'd like help deploying it, I'm available!)

[0] http://www.ganeti.org/

We're using Joyent's Triton platform and like it a lot.

It's been super stable and great for us. Plus #smartos has been incredibly responsive and helpful with any problems we run into.

There is also Proxmox and Cloudstack.

There are some, but in practice I suspect the real competition comes from the major public cloud services in one direction and from more traditional but tried-and-tested IT infrastructure in the other. So far, products aiming for the middle ground, like OpenStack, just haven't quite found their mark. After this much time and investment, in a market where potential is trumped by pragmatism, it's not surprising that major corporate backers might start to look elsewhere for their future strategy.

Cloudstack, OpenNebula - though they are not minimalist.

Full disclosure: I work at OpenNebula.

We believe OpenNebula is a very good alternative to OpenStack. It's a completely open source product, with many strengths. Very easy to setup and to maintain, without the big mess that is required by OpenStack. Plus, users don't get caught in all the politics (lots of vendors pulling in their direction).

I certainly recommend you check it out.

Hmmm, oVirt is probably interesting in this space too:

https://www.ovirt.org

It's more a competitor to VMware clusters though, or was last time I played with it (a few years ago).

It has a decent rep.

So depending on your use case, some of you might be very interested in CloudABI. See: https://nuxi.nl to solve some of your issues. Ed is a FreeBSD committer and has been working on solving some of the harder problems relating to security and god I hate to use this word but "containers" or "containerization" of applications. If you write your app to the CloudABI spec it will run on any of the supported platforms. The video of his presentation at the 32C3 conference does a much better job explaining it. It's certainly worth your time to watch.